Methods of biological control are known for insects and plants. One method currently employed for the control of insect populations is termed the “sterile insect technique” (SIT), also known as the “sterile insect release method” (SIRM). In this method, sterile males are released into the environment, wherein they compete with the wild-type (fertile) males for mates. Females which mate with sterile males produce no offspring, and the release of large numbers of sterile males, therefore, leads to a decrease in the size of the next generation. In this way the size of the wild population is controlled.
SIT requires some mechanism for insect sterilisation. In addition, SIT commonly also employs separation of males from females, with the release of only one sex. This is desirable in the case of an agricultural pest, such as the medfly, where the female damages fruit, even if the female is sterile. Similarly, only the female mosquito bites humans. As such, release of the female insect is preferably avoided in these cases.
Current techniques to achieve both sterilisation and separation of the sexes all have drawbacks. In some cases it is possible to separate males and females by criteria such as pupal mass or time of eclosion, but these methods are unlikely reliably to yield a truly single-sex population. Separation of males and females often involves the use of mutant strains, which have been mutagenised to induce a visible or otherwise selectable difference between the sexes, but such mutagenesis can reduce the fitness of the resultant stock with respect to the wild type, which is undesirable.
Fitness may be further reduced in the sterilisation procedure, in which insects are given a sterilising dose of radiation (X rays or gamma rays), or are chemically sterilised. Frequently, the doses of chemicals or the dose of radiation required to induce sterilisation are very similar to that which is lethal for the organism. As such, sterile organisms are frequently impaired in their ability to mate. Furthermore, both chemical and irradiation methods utilise technologies which are not specific to the target organism, with consequent potential danger to workers. Both methods produce an environmental hazard, as the irradiation source or chemicals will need to be disposed of. In addition, there are inherent dangers and additional labour costs in the use of an irradiation source such as a strontium source.
Fryxell and Miller (Journal of Economic Entomology, Vol 88, No 5, pages 1221-1232) disclose an alternative strategy for insect control, using Drosophila containing a dominant conditional lethal gene which is expressed under appropriate cold conditions in the wild. However, this method can be ineffective due to varying field conditions, where the environment does not provide suitably cold conditions. Moreover, organisms that live in a range of temperature habitats may not be controlled under all conditions.
Asburner et al., (Insect Molecular Biology, 1998, 7(3), 201-213) disclose methods of transformation of insect species with foreign DNA, to produce transgenic species.
DeVault et al. (Biotechnology, Vol 14, January 1996, page 46-49) disclose a two-stage process which is a modification of the SIT procedure. Insects are initially separated by expression of a stably inserted female specific promoter linked to a lethal gene, which is expressed to kill females and to produce just one sex. The remaining males can then be sterilised by irradiation or chemical treatment and released into the environment. However, this method suffers from the drawback referred to above, in that released flies have reduced fitness due to the sterilisation treatment. Alternatively, the DeVault article discloses use of this genetic sexing step in combination with a second genetic system, which may serve to sterilise or retard the hardiness of the natural population.
There is still a need in the art for a method of biological control which avoids the problems with the above methods.
The present invention sets out to overcome such problems.